Active matrix type electroluminescence display device

ABSTRACT

An active matrix type electroluminescence display device comprises a plurality of display pixels GS 11 , GS 12 , GS 13 , etc. arranged in a matrix of rows and columns, each display pixel including an EL element, a first thin film transistor in which a display signal is applied to the drain and which is switched on and off in response to a select signal, a capacitance with one end connected to the source of the first thin film transistor for maintaining a voltage corresponding to the display signal, and a second thin film transistor for driving the EL element based on the display signal. The other ends of the capacitance of row display pixels are connected to and shared by a plurality of first capacitance lines HLA 1 , HLA 2 , HLA 3 , HLAi. Both ends of the plurality of first capacitance lines HLA 1 , HLA 2 , HLA 3  are connected to and shared by second capacitance lines HLB 1  and HLB 2 . A constant voltage is supplied to the second capacitance line.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an active matrix type EL display devicewith display pixels including an electroluminescence element(hereinafter referred to as an EL element) and a thin film transistorarranged in a matrix form, and particularly to an art for stablyilluminating each display pixel by preventing voltage drops incapacitance lines connected to, and shared by, the display pixels.

2. Description of the Related Art

EL elements have various advantages, including, because they are selfilluminating elements, an obviated need for a backlight as required inliquid crystal display devices and unlimited viewing angle. Because ofthese advantages, it is widely expected that EL elements will be use inthe next generation of display devices.

Two basic methods are known for driving EL elements. One of these iscalled a simple, or passive, matrix type, with the other, which employsa thin film transistor as a switching element, is known as an activematrix type. The active matrix type does not suffer from cross talkbetween the column and row electrodes, which is a problem known in thesimple matrix type. Moreover, because the EL elements are driven with alower current density, a high luminescence efficiency can be expected.

FIG. 3 is a circuit diagram schematically showing an active matrix typeEL display device. In the figure, the display pixels GS1, GS2, GS3, . .. GSj are arranged in one row. One display pixel GS1 includes an organicEL element 11, a first thin film transistor 12 (an N channel typetransistor) acting as a switching element in which a display signalDATA1 is applied to the drain and which is switched on and off inresponse to a select signal SCAN, a capacitance 13 which is charged bythe display signal DATA1 supplied when the first thin film transistor 12is switched on and which maintains a maintenance voltage Vh when thefirst thin film transistor 12 is switched off, and a second thin filmtransistor 14 (a P channel type transistor), with its drain connected toa drive supply voltage Vdd and its source connected to the anode of theorganic EL element 11, for driving the organic EL element when themaintenance voltage Vh is supplied from the capacitance 13 at the gate.

The other display pixels GS2, GS3, GSj have an equivalent structure.Although the display pixels are also arranged in the column direction,this arrangement is not shown in the figure in order to simplify thedrawing. Reference numeral 15 represents a gate signal line which isconnected to and shared by each of the display pixels GS1, GS2, GS3, . .. GSj for supplying a select signal SCAN. Reference numeral 16represents a gate drive circuit for supplying the select signal SCAN tothe gate signal line. Reference numeral 17 represents a capacitance linewhich is connected to and shared by the capacitance 13 of each of thedisplay pixels.

The select signal SCAN becomes H level during a selected one horizontalscan period (1H), and the first thin film transistor 12 is then switchedon based on the select signal. Next, a display signal DATA1 is suppliedto one end of the capacitance 13 and the capacitance 13 is charged witha voltage Vh corresponding to the display signal DATA1. The voltage Vhis maintained in the capacitance 13 for a period of one vertical scanperiod (1V) even after the first thin film transistor 12 is switched offdue to the select signal SCAN becoming L level. Because this voltage issupplied to the gate of the second thin film transistor 14, the secondthin film transistor 14 becomes continuous in response to the voltage Vhand the organic EL element 11 is illuminated.

However, in larger size conventional EL display devices, differences inluminance throughout the display device have been observed.

The capacitance line 17 is formed from chrome evaporated on a glasssubstrate, in consideration of heat endurance and ease of processing.Because the capacitance line 17 is extended on the display region inorder to be connected to and shared by each of the display pixels GS1,GS2, GS3, . . . GSj, a resistance and a floating capacitance areinevitably generated. For example, in an active matrix type EL displaydevice having a number of pixels of 220×848, the resistance value of onecapacitance line 17 is approximately 320 Ω and the floating capacitanceis approximately 20 pf. The resistance and floating value increase asthe number of pixels increases.

The capacitance line 17 must be kept constant because it acts as areference potential for charging the display signal DATA1. However, whenthe resistance value of the capacitance line 17 is large, the potentialof the capacitance line 17 becomes unstable when the active matrix typeEL display device is driven, causing a problem that the EL element 11 isnot illuminated at a luminance corresponding to the display signalDATA1. In other words, a select signal SCAN having an H level issupplied to the gate signal line 15 based on the select signal SCAN andthe display signal DATA1 is supplied to one end of the capacitance 13.This causes the display signal DATA1 to be applied to the capacitance 13and the capacitance 13 is charged. If the resistance of the capacitanceline 17 is large, the potential would vary.

SUMMARY OF THE INVENTION

The present invention ensures precise illumination of each display pixelin response to the display signal by supplying a constant voltage fromboth ends of the capacitance line 17 connected to and shared by each ofthe display pixels to stabilize the potential of the capacitance line17.

According to one aspect of the present invention, there is provided anactive matrix type EL display device comprising a plurality of displaypixels arranged in a matrix of rows and columns, each of the displaypixels including an EL element and a capacitance for maintaining avoltage corresponding to a display signal, and a plurality ofcapacitance lines extending to each row and each of which is connectedto and shared by the capacitance of the display pixels, wherein aconstant voltage is supplied from both ends of the capacitance lines.

With this structure, because a constant voltage is supplied from bothends of the capacitance lines, voltage drops in the capacitance linescan be prevented, the potential of the capacitance lines can bestabilized, and, thus, the EL element of the display pixels can beprecisely illuminated in response to the display signal.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a figure illustrating an active type electroluminescencedisplay device according to one embodiment of the present invention.

FIG. 2 is a circuit diagram illustrating a gate drive circuit accordingto the embodiment of the present invention.

FIG. 3 is a diagram illustrating a conventional active type EL displaydevice.

DESCRIPTION OF THE PREFERRED EMBODIMENT

An active matrix type EL display device according to a preferredembodiment of the present invention is described hereinafter referringto FIGS. 1 and 2.

FIG. 1 is a circuit diagram schematically showing a structure of anactive matrix type EL display device. Display pixels GS11, GS12, GS13, .. . GSij, are arranged in rows and columns to form a matrix. Each of thedisplay pixels includes an organic EL element 1, a first thin filmtransistor 2 in which a display signal DATAj is applied to the drain andwhich is switched on and off in response to a select signal suppliedfrom a gate signal line GLi, a capacitance 3, and a second thin filmtransistor 4 for driving the EL element 1 based on the display signalDATAj.

One end of the capacitance 3 is connected to the source of the firstthin film transistor 2. The capacitance 3 is charged with a voltagecorresponding to the display signal DATAj applied to the drain of thefirst thin film transistor and the voltage is maintained. The other endof the capacitance 3 is connected to, and shared by, a plurality offirst capacitance lines HLA1, HLA2, HLA3, . . . extending in each row.Both ends of the first capacitance lines HLA1, HL2, HLA3, . . . areinterconnected by second capacitance lines HLB1 and HLB2. Each of thesecond capacitance lines HLB1 and HLB2 which forms a net of capacitancelines is pulled out to one side of the display region. The secondcapacitance lines HLB1 and HLB2 are interconnected and a constantvoltage Vsc is applied. The first and second capacitance lines areformed from chrome evaporated on a glass substrate. The capacitancelines have large resistance values, but, because a constant voltage Vscis applied via the second capacitance lines HLB1 and HLB2 to the firstcapacitance lines HLA1, HLA2, HLA3, . . . from both sides, a low overallwiring resistance can be achieved for the capacitance lines, and thus,voltage drop can be prevented. Therefore, each capacitance 3 can beuniformly and sufficiently charged with a voltage corresponding to thedisplay signal DATAj. Moreover, even in an organic EL element with ashort illuminating time, a voltage corresponding to the display signalDATAj can be maintained, and thus, the illuminating time of the organicEL element can be extended and stable luminance can be obtained.

FIG. 1 shows a full-color EL display device in which three types ofdisplay pixels are repeatedly arranged, each type of display pixelhaving an organic EL element illuminating respectively in red (R), green(G), and blue (B). In other words, a common drive voltage source RPVddis supplied to the display pixels GS11, GS21, GS31, . . . GSi1 havingorganic EL elements illuminating in red, a common drive voltage sourceGPVdd is supplied to the display pixels GS12, GS22, GS32, . . . GSi2having green illuminating organic EL elements, and a common drivevoltage source BPVdd is supplied to the display pixels GS13, GS23, GS33,. . . GSi3, for blue illuminating organic EL elements. A monochrome ELdisplay device can be constructed by arranging display pixels of onetype in rows and columns.

A display signal DATA1 is applied to the display pixels arranged in thefirst column such as GS11, GS21, and GS31; a display signal DATA2 isapplied to the display pixels arranged in the second column such asGS12, GS22, and GS32; and so on, such that a display signal DATAj isapplied to the display pixels arranged in the jth column such as GS1 j,GS2 j, and GS3 j. A common gate signal line GL1 is connected to thedisplay pixels arranged in the first row such as GS11, GS12, and GS13; acommon gate signal line GL2 is connected to the display pixels arrangedin the second row such as GS21, GS22, and GS23; and so on such that acommon gate signal line GLi is connected to the display pixels arrangedin the ith row such as GSi1, GSi2, and GSi3.

FIG. 2 is a circuit diagram showing a structure of a gate drive circuit5. Shift registers SR1 through SR220 are serially connected forsequentially shifting a reference clock CVK supplied from outside by onehorizontal scan period (1H). The select signal SCAN, which is the outputof each of the shift registers, is transmitted to each of the gatesignal lines GL1 through GL220 via buffer amplifiers 7.

In other words, each of the select signals SCAN having a pulse width ofone horizontal scan period (1H) is shifted by each of the shiftregisters SR1 through SR220 and is output sequentially on each of thegate signal lines GL1 through GL220. To correspond to the number ofpixels of 220×848 in the active matrix type EL display device in thepresent example, 220 shift registers are provided in the embodiment.However, the number of shift registers and buffer amplifiers can bemodified to suit and correspond to the number of pixels.

The active matrix type EL display device is driven as follows. When agate signal line GL1 is selected by a select signal SCAN, the displaypixels in the first row such as GS11, GS21, and GS31 are selected. Atthis point, the gate signal line GL1 is increased to the H level.

During one horizontal scan period (1H), display signals DATA1, DATA2,DATA3, . . . DATAj are sequentially supplied to each of the displaypixels GS11, GS12, GS13, . . . GS1 j from each of the data lines. Thedisplay signals DATA1, DATA2, DATA3, . . . DATAj are maintained by asampling circuit (not shown) and the timing for outputting the signalsis controlled via a transfer gate provided for each of the displaysignal terminals. Because the potential of the first capacitance linesHL1, HL2, HL3, . . . is stabilized in the present invention, thecapacitance 3 can be charged to correspond to the display signals DATA1,DATA2, DATA3, . . . DATAj, in each of the display pixels GS11, GS12,GS13, . . . Gs1 j. Each of the EL elements 1 can be illuminated at itsproper luminance. Similarly, gate signal line GL2 is selected by thenext select signal SCAN. These steps are repeated for one vertical scanperiod (1V).

As described, according to the present invention, the resistance valueof one capacitance line can be reduced by supplying a constant voltagefrom both ends of the capacitance lines. In this manner, the potentialof the capacitance line can be stabilized and the EL element of eachdisplay pixel can be precisely illuminated in response to the displaysignals.

1. An active matrix type electroluminescence display device comprising:a plurality of display pixels arranged in a matrix of rows and columns,each of said display pixels including an electroluminescence element towhich one end of a capacitor for maintaining a voltage corresponding toa display signal is connected via a driver transistor; and a pluralityof capacitor lines extending in a row direction and connected to andshared by the other end of said capacitors of said display pixels;wherein a constant voltage is supplied from both ends of said capacitorlines; said capacitor is connected to a gate of the driver transistor,which drives the electroluminescence element.
 2. An active matrix typeelectroluminescent display device comprising: a plurality of displaypixels, each including an electroluminescent element, arranged in amatrix of rows and columns, a first thin film transistor in which adisplay signal is applied to the drain and which is switched on and offin response to a select signal, a capacitor having one end connected tothe source of the first thin film transistor and for maintaining avoltage corresponding to said display signal, and a second thin filmtransistor for driving said electroluminescence element based on saiddisplay signal; a plurality of first capacitor lines, each extending ina row direction and connected to and shared by the other end ofcapacitors of said display pixels; a second capacitor line connected tofirst ends of said plurality of first capacitor lines; a third capacitorline connected to second ends of said plurality of first capacitorlines; wherein said second and third capacitor lines are connected to acommon constant voltage source, and said constant voltage is supplied tosaid first ends and said second ends of said plurality of firstcapacitor lines through said second and third capacitor lines.
 3. Thedevice of claim 2, wherein said second capacitor line extends in acolumn direction on one side of an area in which said plurality ofdisplay pixels are arranged in a matrix, and said third capacitor lineextends in a column direction on the other side of the area in whichsaid plurality of display pixels are arranged in a matrix.
 4. An activematrix type electroluminescence display device comprising: a pluralityof display pixels, each including an electroluminescence element,arranged in a matrix of rows and columns, a first thin film transistorin which a display signal is applied to the drain and which is switchedon and off in response to a select signal, a capacitor having one endconnected to the source of the first thin film transistor and formaintaining a voltage corresponding to said display signal, and a secondthin film transistor for driving said electroluminescence element basedon said display signal; a plurality of first capacitor lines, eachextending in a row direction and connected to and shared by the otherend of capacitors of said display pixels; a second capacitor lineconnected to first ends of said plurality of first capacitor lines; athird capacitor line connected to second ends of said plurality of firstcapacitor lines; and wherein a constant voltage is supplied to saidfirst ends and second ends or said plurality of first capacitor linesthrough said second and third capacitor lines.
 5. The device of claim 4,wherein said second capacitor line extends in a column direction on oneside of an area in which said plurality of display pixels are arrangedin matrix, and said third capacitor line extends in a column directionon the other side of the area in which said plurality of display pixelsare arranged in matrix.
 6. The device of claim 1 comprising: a secondcapacitor line connected to first ends of said plurality of capacitorlines; a third capacitor line connected to second ends of said pluralityof capacitor lines; and wherein said constant voltage is supplied tosaid first ends and second ends or said plurality of capacitor linesthrough said second and third capacitor lines.